Both are wonderful. Skyrim has a much more interesting flora and fauna system and is just beautiful to travel. FNV has better companions and overall writing. I thought Skyrim's opponents were much more real, as were their motivations. To me, the Imperials vs the Stormcloaks was more realistic than Kaisar's legions vs NCR. The chance of the legion winning against the NCR just didn't make sense. The House side of FNV was just silly and out of place. Then again, Skyrim had the benefit of seeing what another developer did with their engine, and I think it paid off.